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Old 6th November 2012, 14:37   #46
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Default Re: About Rocket Science & Engines

Any specific reason why rockets are built in stages? Will a single piece rocket have to be a lot bigger to achieve the same results as a multi-stage one, or perhaps it can't achieve the same speed? Am I right in assuming each stage as it takes off from where the previous one left off, achieves more speed, same way as a rifle bullet fired from a fighter plane?
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Old 6th November 2012, 14:59   #47
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Originally Posted by Gansan View Post
Any specific reason why rockets are built in stages? Will a single piece rocket have to be a lot bigger to achieve the same results as a multi-stage one, or perhaps it can't achieve the same speed? Am I right in assuming each stage as it takes off from where the previous one left off, achieves more speed, same way as a rifle bullet fired from a fighter plane?
Weight is one of the biggest factors when you are launching anything to space. When you have different stages, as soon as their work is done/fuel is depleted, the whole stage can be detached and hence reduces weight for the next stage to push forward.
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Old 6th November 2012, 15:08   #48
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Originally Posted by Gansan View Post
Any specific reason why rockets are built in stages? Will a single piece rocket have to be a lot bigger to achieve the same results as a multi-stage one, or perhaps it can't achieve the same speed? Am I right in assuming each stage as it takes off from where the previous one left off, achieves more speed, same way as a rifle bullet fired from a fighter plane?
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Weight is one of the biggest factors when you are launching anything to space. When you have different stages, as soon as their work is done/fuel is depleted, the whole stage can be detached and hence reduces weight for the next stage to push forward.
Well @Gansan sir, SunnyBoi sir answered your question.

1. Adding to it, as you said, the relative velocity achieved is similar to a missile fired from a fighter jet (more than a bullet - as bullet will fly just by impulse and not with its own propulsion source - whereas multiple stages mean - multiple propulsion systems).

2. Staging gives the flexibility to utilize the same rocket for different applications, like Soyuz launcher, can be configured to launch astronauts into space or carry unmanned satellites into space. Better example is PSLV series - Standard, Core Alone (CA), XL (extended) (chandrayan launcher).
Probably the image below gives you a fair idea. (out of these variants 3S and HP are designed for GAGAN/IRNSS payloads - now that might be launched using a GSLV instead of PSLV. Hence, 3S and HP variants exists only in theory.)

http://1.bp.blogspot.com/_JRgHKYCaeH...Bvariants1.jpg


Last edited by AlphaKilo : 6th November 2012 at 15:11. Reason: grammer
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Old 6th November 2012, 18:25   #49
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Originally Posted by AlphaKilo View Post
Seperate post coming up soon!

I will answer your second question first.
Thanks a lot, understood the concept. Apologies for accelerating the satellites close to the earth, what I meant was actually decelerating them.

Would lookout for second post on the first question.
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Old 7th November 2012, 00:18   #50
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Originally Posted by CLIX View Post
ALERT!

I am not sure if you would like to answer this as part of this thread - but what is your take on Objects sighted while in space from the spacecrafts, that are many times discounted as space junk, dead pixels or lens flare among other things, but have been known to move in strange trajectories and varying velocities
The moment I use the three letter acronym its rubbished away as fantacy - I shall let you take it from here.
Answer for your question is even discussed world wide! You seem to have created quite a lot of stir.

http://www.telegraph.co.uk/news/news...fter-all.html#

Quote:
For decades, they have been scanning the skies for signs of alien activity.

But having failed to establish any evidence for the existence of extraterrestrial life, Britain’s UFO watchers are reaching the conclusion that the truth might not be out there after all.

Enthusiasts admit that a continued failure to provide proof and a decline in the number of “flying saucer” sightings suggests that aliens do not exist after all and could mean the end of “Ufology” – the study of UFOs – within the next decade.
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Old 23rd November 2012, 10:49   #51
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Yeah... folks seem to be loosing interest in the UFO space!

Is it possible that the super-powers too used this cover to test / develop their classified programmes? Great cover - UFO: No idea what you are talking about; Being investigated; Can neither deny nor accept if this ever happened
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Old 23rd November 2012, 13:43   #52
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Is it possible that the super-powers too used this cover to test / develop their classified programmes? Great cover - UFO: No idea what you are talking about; Being investigated; Can neither deny nor accept if this ever happened
Well, that's an old trick now a days. Given the amount of space based surveillance, its hard to hide stuff flying around anymore! Probably even the aliens are much worried about the chaos on earth and are bored by all the drama happening around and might have left!

By the way, a long post on navigating in the space and deciding orbits is long due, will put it up in the coming week, bear with me!
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Old 23rd November 2012, 23:37   #53
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By the way, a long post on navigating in the space and deciding orbits is long due, will put it up in the coming week, bear with me!
I almost wanted to remind you, good you are working at it. We understand it is a long haul nav, don't put yourself under any pressure, let it chart its course whenever ready.

BTW who is that guy in your avatar? If you don't mind sharing.

Last edited by PGA : 23rd November 2012 at 23:40.
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Old 26th November 2012, 09:52   #54
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BTW who is that guy in your avatar? If you don't mind sharing.
The Red Baron AK? ... if I may? A WWI Ace German fighter pilot with some crazy number of victories in the air. [or his twin borther ]
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Old 26th November 2012, 10:30   #55
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Originally Posted by CLIX

The Red Baron AK? ... if I may? A WWI Ace German fighter pilot with some crazy number of victories in the air. [or his twin borther ]
Baron Manfred Von Richtofen. Credited with in excess of 70 or 80 confirmed 'kills' in WW1.
He was called the Red Baron and was an ace pilot, aristocrat and gentleman as far as I have read.
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Old 26th November 2012, 12:25   #56
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Originally Posted by PGA View Post
I almost wanted to remind you, good you are working at it. We understand it is a long haul nav, don't put yourself under any pressure, let it chart its course whenever ready.

BTW who is that guy in your avatar? If you don't mind sharing.
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Originally Posted by CLIX View Post
The Red Baron AK? ... if I may? A WWI Ace German fighter pilot with some crazy number of victories in the air. [or his twin borther ]
Quote:
Originally Posted by shankar.balan View Post
Baron Manfred Von Richtofen. Credited with in excess of 70 or 80 confirmed 'kills' in WW1.
He was called the Red Baron and was an ace pilot, aristocrat and gentleman as far as I have read.
Thankyou Gentlemen!

As said above, he is the German ace of WW1 - Manfred von Richthofen aka. Red Baron (as called by his foes).

http://en.wikipedia.org/wiki/Manfred_von_Richthofen

@PGA - For sure, I am currently working on how to cut short the whole navigation (mathematics) behind it so that it is understandable by everyone. Hence give me some time, I will surely come up with all the details.
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Old 24th January 2013, 15:02   #57
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Quote:
Originally Posted by PGA View Post
I almost wanted to remind you, good you are working at it. We understand it is a long haul nav, don't put yourself under any pressure, let it chart its course whenever ready.

BTW who is that guy in your avatar? If you don't mind sharing.
Sir, here you go! Finally I am writing the answer to your question. (Sorry about the longg delay!)

I believe everyone is aware of the orbits we have around the earth and the needs and uses of the different orbits. With this knowledge, let us now start off in small steps. Here I will write about how to orbit around the earth, followed by reaching to moon, which will be finally followed with reaching other planets or any where in deep space.

Orbiting around the earth:

As far as rocket science or space technology is concerned, this is by-far the most easiest of all yet trickiest of all. First of all, it is easy because we have been doing it for over 60 years now and we have our maximum experience here.

The basic need to reach the orbit is the speed/velocity to first escape out of the gravitational pull, and that speed should be good enough not to escape out of the sphere of influence* of the earth.

* - A sphere of influence is an assumed sphere around the globe within which the earth's magnetic field and gravity field still have their influence.

In-order to be able to escape the atmosphere, we use rockets, which act like a transport system for the satellites to reach the desired orbit. These rockets are designed to reach the magic number of 11.8 m/s (escape velocity) which will propel any object out of the earth's gravity pull. Once after escaping the atmosphere, the satellite is either seperated from the transport rocket or is injected into its orbit by a mini-rocket attached to the satellite. The momentum (energy with which the object was propelled into this motion) is carried forward and is used to reach the desired altitude.

The calculation goes like this (I am not putting any formulas in here to keep the whole point simple) - the velocity (momentum) necessary to reach this desired orbit is pre-calculated (during design of rocket) based on the weight of the satellite and the required positioning accuracy. This means, the rocket will not provide anything more than necessary for the satellite to reach its destination. If any mistake happens, there is no fall-back or plan B. But to save weight and reduce costs, everything has to be on a strict diet and needs to calculated to very high order of precision.

Once the after reaching the desired orbit, the satellites revolve around the earth based on the centrifugal force, i.e. earth trying to pull them towards itself whereas the moment that it has carried over the launch will try to pull it away, thus keeping it in its desired position because, Earth's pull is stronger! (Universal law of gravity - GMm/R≤, where small m = mass of object and M = mass of earth, R = distance between the objects) and in most cases the small m can be comfortably ignored!

Now comes the question, if the pull is so large, won't the satellites simply not fall back towards the earth. If to answer this simply, then, YES, they do fall down with time, but as we learned earlier, the attitude and orbit control system performs corrections to keep the satellite in its orbit and altitude. International Space Station performs "Orbit raising" maneouvres quite often to raise the orbit of the spacestation.

This was simple and easy to do when we operate in LEO, what to do for GEO/MEO? There are following options:

1. Have a large rocket, which will thrust all the way to its final location at 36000 kms (GEO).

2. Reach an intermediate orbit (GTO - no, not our dear Moderator! ), and then use low thrust methods and spiral around the earth several times to perform some boost maneouvres to reach the final altitude.

There are advantages and disadvantages in both methods. In the former, although its quite fast and simple to reach directly, but, designing and developing such a rocket will become an costly affair. As discussed earlier, the fuel /payload ratio will be so high that it won't be cost effective anymore. Hence, the second method, an compromise between cost and time! This uses smaller rockets, and the small rocket(tiny rockets in reality) thrust the satellite when they reach the perigee, thereby slowly raising the apogee and reaching the final destination. The raise in apogee will be such that it reaches the the altitude aligns itself with the destination altitude/orbit.

The direct transfer from a lower ellipse to an higher ellipse is achieved by something called as an "Hohmann Transfer". This is considered the most energy efficient and the easiest of the transfers possible.

Quote:
Calculation

For a small body orbiting another, very much larger body (such as a satellite orbiting the earth), the total energy of the body is the sum of its kinetic energy and potential energy, and this total energy also equals half the potential at the average distance , (the semi-major axis):
Solving this equation for velocity results in the Vis-viva equation,
where:
  • is the speed of an orbiting body
  • is the standard gravitational parameter of the primary body, assuming is not significantly bigger than (which makes )
  • is the distance of the orbiting body from the primary focus
  • is the semi-major axis of the body's orbit.
Therefore the delta-v required for the Hohmann transfer can be computed as follows, under the assumption of instantaneous impulses:
, , where and are, respectively, the radii of the departure and arrival circular orbits; the smaller (greater) of and corresponds to the periapsis distance (apoapsis distance) of the Hohmann elliptical transfer orbit. The total is then:
Whether moving into a higher or lower orbit, by Kepler's third law, the time taken to transfer between the orbits is:
(one half of the orbital period for the whole ellipse), where is length of semi-major axis of the Hohmann transfer orbit
Source(above): wiki
source (Image and quote): http://jwilson.coe.uga.edu/emt668/em...on/image45.gif
This looks something like this:


Quote:
To understand Hohmann Transfer Orbit consider the diagram alongside. A vehicle is traveling in some orbit A around the Earth and we want to get it to orbit C. At some point the engine performs a posigrade burn thus enlarging the orbit and the vehicle is now traveling along orbit B. The point where the posigrade burn took place becomes the point of perigee of the new orbit (B). Unless there is a further burn the vehicle will now continue to move in orbit B. Since we want to move the vehicle to orbit C, the size of the posigrade burn (at perigee) was well designed to ensure that the point of apogee (of orbit B) meets orbit C. At apogee a further posigrade burn is used to enlarge the orbit again this time the vehicle goes into orbit C and the transfer is complete.
Rest: continues in the next post!

Last edited by AlphaKilo : 24th January 2013 at 15:09.
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Old 24th January 2013, 15:26   #58
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Well, you see, its time for some 10+2 physics lessons. The earlier post was just an introduction into orbit manoeuvrings et. al. Now before I go deeper into it, its time we brush up the basics a little, like hearing some familiar names - Kepler, Newton ( I hate this guy! he shows up everywhere!!! why can't I be famous like him?) and so on.

First the basic idea behind the orbits was formed by Johannes Kepler, way back in the 16th century (1609). Simply put down, they are:
Source: Wiki
Quote:
Kepler's laws are:
  1. The orbit of every planet is an ellipse with the Sun at one of the two foci.
  2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.
All three laws in the form of a diagram:



Quote:
(1) The orbits are ellipses, with focal points ƒ1 and ƒ2 for the first planet and ƒ1 and ƒ3 for the second planet. The Sun is placed in focal point ƒ1. (2) The two shaded sectors A1 and A2 have the same surface area and the time for planet 1 to cover segment A1 is equal to the time to cover segment A2. (3) The total orbit times for planet 1 and planet 2 have a ratio a13/2 : a23/2.
So, what have we got to know from the above laws:
Source:http://jwilson.coe.uga.edu/emt668/em..._unit_two.html
Quote:
From Kepler's laws we get the following:
The planets revolve around the Sun in elliptical orbits with the Sun at one focus of the orbit.
This statement can be generalized to explain the motion of satellites about the Earth and indeed it can be generalized to explain the behavior of all the objects traveling in the solar system. This is quite clearly a simplification - there are a number of things this definition does not account for such as collisions etc. - but for our purposes it is adequate! In fact Kepler's law more accurately states that:
If two objects interact gravitationally, each will describe an orbit that is a conic section out of the common mass of the pair. If the objects are permanently associated, their orbits will be ellipses. If they are not permanently associated with each other, their orbits will be hyperbolas.
The latter part of this law explains that comets or similar objects have hyperbolic orbits and visit the sun only once.
In the coming days, we will learn that not all of the above is 100% true! More on that later.

Now to some terminologies:


Source: http://jwilson.coe.uga.edu/emt668/em...coordinate.gif

To enable discussion about Earth centered orbits a coordinate system called the Earth-centered coordinate system has been developed. The center of the Earth is regarded as the origin, the x-y plane contains the equator and is called the Earth's equatorial plane, the positive x-axis points in the direction of the vernal equinox (the place where the Sun's orbit is in line with the equator as the Sun moves from the northern hemisphere to the southern hemisphere).
Some resulting vocabulary
  • Perigee- the point in the elliptical orbit that is closest to the focus (in sun-centered orbits this is called the perihelion)
  • Apogee - the point in the elliptical orbit that is farthest from the focus (in sun-centered orbits this is called the aphelion)
  • Ascending node - the point where the orbit moves from below to above the equatorial plane,
  • Descending node - the point where the orbit moves from above to below the equatorial plane,
  • Line of nodes- the line joining the ascending and descending node,
  • Posigrade orbit - an orbit that moves in a counterclockwise direction if viewed looking down from the north pole, and
  • Retrograde orbit - an orbit that moves in a clockwise direction if viewed looking down from the north pole.
Six orbital elements now completely determine the parameters of the orbit.
  • a - the length of the semi major axis (i.e. half of the major axis)
  • e - the orbital eccentricity (e = c/a where c is the distance from the center of the ellipse to the focus)
  • i - the angle of inclination of the orbit with respect to the equatorial plane
  • omega - the angle formed by the positive x-axis and the line from the origin to the ascending node
  • argument of perigee- the angle in the orbital plane formed by the ascending node, the origin and perigee
  • t- the time of perigee passage - that moment when the orbiting object passes perigee.
Source:http://jwilson.coe.uga.edu/emt668/em..._unit_two.html

Edit: For all those Geeks and nuts over here, who have questions on why and how and trying to compete with Einstein and Newton, please see this link:

http://www.amsat.org/amsat/keps/kepmodel.html

Last edited by AlphaKilo : 24th January 2013 at 15:35. Reason: Added a link for more information
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Old 25th January 2013, 18:23   #59
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So far to the basics of Keplerian laws and terminologies. Now let us go back to the topic of navigating in space.
Further to the Hohmann transfer orbit, it is also possible to spiral around and reach the final destination. This is nothing but several Hohmann transfers done at a very low delta V (thrust impulse/firing rockets producing very low thrust).
Means like a cork screw, the satellite simply spirals out/in towards its target. The basic question while choosing the orbit is the mode of propulsion available and the weight budget. If the satellite is big enough and weight allowances are
wide enough, then an chemical propulsion ala. Hohmann transfer is chosen. But off-late there is a world-wide gaga over "kitna deta hain" and hence, people are switching to alternate propulsion mechanisms such as electrical propulsion.
Although electrical propulsion system provide very very low thrust, but they are capable of offering very high delta V's, that is very very long thrust duration and hence, final velocity achieved will be comparatively higher. Further, the whole
system set-up weighs very less. Finally, since its thrust is very low, very very precise orbit manouvres can be performed and pointing/directional accuracy achieved is very amazing.

The only advantage of the chemical propulsion over the electrical system (Hohmann transfer over spiral transfer) is that the overal transfer time is very low in an Hohmann transfer. Why is this important for us? well, commercially, it is necessary
to commission the satellites at the earliest possible time after launch (TV satellites) and there is this middle zone between LEO and GEO which hosts something called as "Van-Allen belts" containing harmful radiative particles and some meteorite debris
which could damage the electronics in the satellite. Hence, it is absolutely necessary to cross this zone asap.
Little bit of mathematics to prove my point:Source: http://ccar.colorado.edu/asen5050/pr...9/Miller_Proj/

Quote:
In order to compare the performance of electric propulsion to that of other propulsion methods, the performance of these other methods must first be described. A conventional orbital maneuver entails a series of instantaneous ΔV burns to complete its transfer. In a maneuver known as a Hohmann transfer to ΔV burns are performed to first take the satellite from a circular orbit to an elliptical transfer and then from the transfer orbit back to a circular at a different altitude. To find the value of these burns, the initial and final orbital velocities must be found. These velocities are described by: (1)
(2)
Where Vi is the initial velocity, Vf is the final velocity, μ is gravitational parameter for Earth, Ri is the initial orbital radius and Rf is the final orbital radius. Now that the starting and ending velocities are known, the parameters for the transfer orbit must be found. The semi-major axis of the transfer orbit and the eccentricity of the transfer orbit are seen in Equations 3 and 4.
( 3)
(4)
Knowing the semi-major axis of the transfer orbit allows for the velocity to be calculated in the transfer orbit at the distance Ri and Rf. Equation 5 shows what the velocity at an orbital radius of R must be in order to maintain the transfer orbit.
(5)
The ΔV burns can now be quantified using the values found in Equations 1, 2 and 5. The change in velocity at both locations of the transfer maneuver can be found using Equation 6 and Equation 7.
(6)
(7)
The quantities found from Equations 6 and 7 are then summed together to form the total change in velocity imparted during the two impulsive burns performed during the maneuver. Two other important aspects to consider in conventional propulsion methods is the transfer time and the amount of fuel consumed during the burns. The first of these is found in Equation 8:
(8)
Where Ttrans is the time it takes to complete the half ellipse transfer orbit. The amount of fuel consumed the two burns is seen in Equation 9:
] (9)
Where m0 is the initial mass of the spacecraft before the burns, ΔV is the increase in velocity imparted, g0 is the acceleration due to gravity on the Earth’s surface and Isp is the specific impulse of the rocket motor. With all of these values determined, the performance and orbital dynamics of the transfer can be evaluated.
Calculating the transfer orbit created by the continuous, low thrust propulsion is more difficult than the Hohmann transfer. Instead of impulsive burns, one continuous burn is performed during the entire transfer. This creates an orbit that spirals from the initial orbit to the final orbit. To model this, a series of differential equations must be created to characterize both the gravity effects and the thrust created by the engine. The four differential equations describing the behavior are seen in Equations 10-13:
(10)
(11)
(12)
(13)
Where T is the thrust, m is the mass of spacecraft, a is the semi-major axis and θ is the true anomaly. These first order differential equations can be solved easily using a numerical integrator such as the one in MATLAB named ode45. The equations yield the velocity, the mass, the semi-major axis and the true anomaly of the spacecraft as a function of time. From these the numerical solution, the change in velocity, propellant mass, transfer period, and profile of the transfer can be found. First the velocity change imparted by the motor as seen in Equation 14 is simply the initial velocity minus the final velocity.
(14)
The propellant mass will just be the initial mass of the spacecraft minus the final mass produced by the differential equation solver as seen below.
(15)
The orbital transfer period is identical to the time produced by the differential equation solver when the satellite reaches its final orbital radius. The profile of the orbit can be plotted by using a polar of the true anomaly and semi-major axis. From the values calculated from both transfer method will now allow for a comparison of the conventional Hohmann transfer and the low thrust transfer.
An example of difference between a Low Thrust Transfer and an Hohmann Transfer:



The source website mentioned above has a very good example, and I request all those who are particularly interested to know more to pay a visit there. If there are more questions, please feel free to ask me here, I will answer to the best of my knowledge.

Just to give your some numbers to play with: Source:http://ccar.colorado.edu/asen5050/pr..._2009/daddino/


Last edited by AlphaKilo : 25th January 2013 at 18:35.
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Old 25th January 2013, 18:38   #60
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Taking the lesson one step further, now lets see how to navigate in deep space. For this, I will be using several actual examples (real missions) and extracts from them to explain how interplanetary/inter-stellar travel works. Just a teaser to start with:


Source: http://sbir.gsfc.nasa.gov/SBIR/succe...s/5-075pic.jpg
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